Heat Transfer MCQ’s – Correlations for Forced Convection
1 - Question
The convective heat transfer coefficient in laminar flow over a flat plate
a) Increases with distance
b) Increases if a higher viscosity fluid is used
c) Increases if a denser fluid is used
d) Decreases with increase in free stream velocity
Explanation: It mostly increases if a denser fluid is used.
2 - Question
For laminar flow over a flat plate, the average value of a Nusselt number is prescribed by the relation Nu = 0.664 (Re) 0.5 (Pr) 0.33 Which of the following is then a false statement?
a) Density has to be increased four times
b) Plate length has to be decreased four times
c) Specific heat has to be increased four times
d) Dynamic viscosity has to be decreased sixteen times
Explanation: The dynamic viscosity has an inverse relation to 1/6 power. To double the convective heat transfer coefficient, the dynamic viscosity has to be decreased 64 times.
3 - Question
For turbulent flow over a flat plate, the average value of Nusselt number is prescribed by the relation Nu = 0.664 (Re) 0.5 (Pr) 0.33 Which of the following is then a false statement? The average heat transfer coefficient increases as
a) 1/5 power of plate length
b) 2/3 power of thermal conductivity
c) 1/3 power of specific heat
d) 4/5 power of a free stream velocity
Explanation: The average heat transfer coefficient reduces with length as 1/5th power of the length.
4 - Question
A nuclear reactor with its core constructed of parallel vertical plates 2.25 m high and 1.5 m wide has been designed on free convection heating of liquid bismuth. Metallurgical considerations limit the maximum surface temperature of the plate to 975 degree Celsius and the lowest allowable temperature of bismuth is 325 degree Celsius. Estimate the maximum possible heat dissipation from both sides of each plate. The appropriate correlation for the convection coefficient is Nu = 0.13 (Gr Pr) 1/3
a) 143 MW
b) 153 MW
c) 163 MW
d) 173 MV
Explanation: Q = 2 h A d t = 153 MW.
5 - Question
Consider the above problem, find the value of Grashoff number
a) 101.3 * 10 12
b) 102.3 * 10 12
c) 103.3 * 10 12
d) 104.3 * 10 12
Explanation: Grashof number = l 3 p 2 β g d t/µ 2.
6 - Question
A thin walled duct of 0.5 m diameter has been laid in an atmosphere of quiescent air at 15 degree Celsius and conveys a particular gas at 205 degree Celsius. Base your calculations on one meter length of the duct, estimate the convective coefficient of heat transfer
a) 5.086 W/m2 K
b) 6.086 W/m2 K
c) 7.086 W/m2 K
d) 8.086 W/m2 K
Explanation: h = 1.37 (d t/l) 0.25 = 5.086 W/m2 K.
7 - Question
Free correction modulus is given by
a) p 2 β g c P/µ
b) p 2 β g c P/k
c) p 2 β g c P/µ k
d) p 2 β g c P
Explanation: It contains only fluid properties and is called the free convection modulus.
8 - Question
The free convection coefficient is given by h = C 1 d t m/l 1 – 3m The value of exponent for laminar flow is
Explanation: For laminar flow h = C 1 (d t/l) 0.25.
9 - Question
For inclined plates we multiply Grashoff number with
a) Cos 2 α
b) Sin 2 α
c) Sin α
d) Cos α
Explanation: It should be multiplied with cos α, as α is angle with the horizontal.
10 - Question
The free convection coefficient is given by h = C 1 d t m/l 1 – 3m The value of exponent for turbulent flow is
Explanation: For turbulent flow h = C (d t).